Hon Hai Precision Industry, better known as Foxconn, reportedly used a group of 3,000 students from Zhengzhou Urban Rail Transit School to help assemble the iPhone X.

Six student interns aged 17-19 reportedly said that they routinely worked 11-hour days at a factory in Zhengzhou, which is illegal under Chinese law, according to The Financial Times.

The students reportedly told The Financial Times that they must complete three months of "work experience" in the factory in order to graduate. Foxconn reportedly said that its internship program was carried out in co-operation with local governments and a number of vocational schools in China.

The Financial Times reported that an 18-year-old student called Ms Yang, who is planning to become a train attendant, said: "We are being forced by our school to work here."

Yang did not want to provide her first name out of fear that she would be punished, according to The Financial Times. She reportedly said that she assembled 1,200 iPhone X cameras a day, adding "This has nothing to do with our studies."

Apple and Foxconn did not immediately respond to Business Insider's request for comment but the companies reportedly told The Financial Times they were aware of student interns working overtime at a factory in China. Both reportedly said they were taking remedial action but added that the students were working voluntarily.

Apple's iPhone X — launched 10 years after the first iPhone — has been plagued by production issues that meant it ended up shipping in November instead of September, which is when Apple usually starts shipping its new iPhones.

A Foxconn employee told The Financial Times that Foxconn employs students every year between August and December to help it cope with demand from Apple. During this period, staff numbers at the Zhengzhou factory can triple from 100,000 to more than 300,000, the employee reportedly told The Financial Times.